High-voltage metal-oxide-semiconductor (HVMOS) devices are widely used in many electrical devices, such as input/output (I/O) circuits, CPU power supplies, power management systems, AC/DC converters, etc. There are a variety of forms of HVMOS devices. For example, a symmetric HVMOS device may have a symmetric structure on the source side and drain side, and high voltage can be applied on both drain and source sides. An asymmetric HVMOS device may have asymmetric structures on the source side and drain side. For example, only one of the source side and drain side, typically the drain side, is designed for sustaining high voltages.
FIG. 1 illustrates a conventional high-voltage MOS device 2, which is also referred to as double-diffused drain (DDD) MOS device 2. DDD MOS device 2 includes gate oxide 10 on P-well region 16, gate electrode 12 on gate oxide 10, and gate spacer 8 on sidewalls of the gate oxide 10 and gate electrode 12. Lightly doped drain region 4 is formed on the drain side and substantially aligned with an edge of gate electrode 12. Heavily doped drain region 6 is formed in the lightly doped drain region 4, and is spaced apart from the edge of gate spacer 8. Source region 14 is heavily doped. The space between the heavily doped drain region 6 and the edge of gate electrode 12 results in an increase in the drain-to-gate breakdown voltage.
Breakdown voltage is a key parameter of HVMOS devices. Increasing breakdown voltage without the cost of additional mask layers is one of the major goals in the design of HVMOS devices. Typically, the breakdown voltage of an HVMOS device is related to its size. For example, the improvement in the breakdown voltage of the structure shown in FIG. 1 may be achieved by increasing the distance between heavily doped drain region 6 and gate electrode 12. However, such improvement requires more chip space. MOS devices having greater sizes often have greater power consumption. Therefore, increasing breakdown voltage by enlarging the size of the HVMOS device is not a desirable design approach. A new HVMOS structure and formation methods are thus needed.